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Each FC storage array port has a maximum queue depth of 2048. For performance reasons we’ll have to do the math with 1600. Suppose a large number of HBAs (initiators) are generating IOs, a specific port queue can fill up to the maximum. The host’s HBA will notice this by getting queue full (QFULL) messages and very poor response times. It depends on the Operating system how this is dealt with. Older OSs could loose access to it’s drives or even freeze or get a blue screen. Modern OSs will throttle IOs down to a minimum to get rid of this inconvenience. VMware ESX for example decreases it’s LUN queue depth down to 1. When the number of queue full messages disappear, ESX will increase the queue depth a bit until it’s back at the configured value. This could take up to around a minute.
During the QFULL events the hosts may experience some timeouts, even if the overall performance of the CLARiiON is OK. The response to a QFULL is HBA dependent, but it typically results in a suspension of activity for more than one second. Though rare, this can have serious consequences on throughput if this happens repeatedly.
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